1. Field of the Invention
The present invention relates to a progressive multifocal lens used to assist an accommodation power of an eye.
2. Related Background Art
As spectacle lenses for assisting an accommodation power when an accommodation power of an eye becomes too weak to see a near point, various progressive multifocal lenses have been known. A progressive multifocal lens of this type has an upper "farsight portion", a lower "nearsight portion" and a progressive region (referred to as an "intermediate portion") which is located between the above two portions in which a refracting power is continuously changed. When the progressive multifocal lens is actually worn as spectacles by a user, the farsight portion becomes an upper portion while the nearsight portion becomes a lower portion. In the present invention, it is to be noted that "upper portion", "lower portion", "horizontal" and "vertical" are used to point positions of the progressive multifocal lens in the state that it is actually worn by the user. For example, the lower portion of the farsight portion is a position within the farsight portion close to the intermediate portion. Also, the difference between the nearsight refracting power and the farsight refracting power is called an additional power.
In a progressive multifocal lens of this type, in general, when wide distinct vision areas of the farsight and nearsight portions are kept and coupled by the progressive power range, lens aberrations are concentrated on side regions of the progressive power range. As a result, blurring and distortion of an image occur in these side region. Therefore, when a user wears spectacles formed of these multifocal lenses and shifts his eyes laterally, he perceives the distortion of an image as the fluctuation of the image has an unpleasant feeling.
In order to prevent such a problem of visual characteristics, in known progressive power lenses, various designs and evaluations have been made from a variety of viewpoints.
FIG. 1 is a schematic plan view showing the divisions of regions of a progressive multifocal lens designed symmetrically. This progressive multifocal lens has a farsight portion F corresponding to the upper portion at the time of actual use, a nearsight portion N corresponding to the lower portion and an intermediate portion P in which the refracting power is changed continuously between the farsight and nearsight portions F and N. As for the shape of a lens surface, the intersecting curve MM' of the object-side lens surface and a cross section along a meridian line passing approximately the center of a lens surface vertically from top to bottom is used as a reference line for expressing specifications such as the additional power of the lens, and is also used as an important reference line in the lens design. In the thus designed progressive multifocal lens, the center OF of the farsight portion F, a farsight eye point E and the center ON of the nearsight portion N are located on the center line MM' as the reference.
Further, as shown in FIG. 2, in consideration of the fact that the nearsight portion N comes close to a nasal side when worn by the user, another progressive multifocal lens has been proposed in which a nearsight portion N is arranged asymmetrically (hereinafter referred to as a "asymmetrical progressive multifocal lens").
Also, in such an asymmetrical progressive multifocal lens, a center line MM' consisting of the intersecting line of an object-side lens surface and a cross section passing the center OF of a farsight portion F, a farsight eye point E and the center ON of a nearsight portion N is used as a reference line. In the present invention, these reference lines will be referred to as "a principal meridional curve" hereinafter.
Conventional progressive multifocal lenses are disclosed in, e.g., U.S. Pat. Nos. 3,687,528, 3,910,691 and Japanese Patent Publication Nos. 59-42285 and 63-42764.
In the lens disclosed in U.S. Pat. No. 3,687,528, as for the shapes of intersecting lines of planes perpendicular to the principal meridional curve and a refracting surface of the lens, only an intersecting line at a point corresponding to substantially the center of an intermediate portion defines a circular arc. Intersecting lines above and below that point define non-circular arcs in which a radius of curvature is decreased as the intersecting line is spaced apart from the principal meridional curve in a portion above the point, and vice versa in a portion below the point. Therefore, only the center of the intermediate portion defines a circular arc, and other portions define simple non-circular arcs. Therefore, distinct vision areas (a range with an astigmatic difference of 0.5 D (diopter) or less) of the farsight and nearsight portions become narrow, and a field of view is also narrowed due to an abrupt change in aberrations, resulting in considerable distortion and fluctuation of an image.
In the lens disclosed in U.S. Pat. No. 3,910,691, the dynamic vision is improved in addition to the static vision in the lens disclosed in U.S. Pat. No. 3,687,528. However, although the dynamic vision can be improved to some extent, it is still difficult to attain a sufficient performance in practical use.
In a lens disclosed in Japanese Patent Publication No. 59-42285, as for the shapes of intersecting lines each defined by a plane perpendicular to the principal meridional curve and a refracting surface of a lens, the intersecting lines define a non-circular arc in the upper portion of the farsight portion. More specifically, in the upper portion of the farsight portion, the radius of curvature of each intersecting line is decreased as it goes away from the principal meridional curve, a rate of decrease in radius of curvature approaches 0 toward an upper peripheral portion and the intersecting lines near the upper peripheral portion have a constant radius of curvature. In the lower portion of the farsight portion, the intersecting lines define non-circular arcs and the radius of curvature of each intersecting line is decreased monotonously. In the intermediate portion, the intersecting lines define non-circular arcs except for a connecting portion with the farsight portion, in which the radius of curvature of each intersecting line is increased and then decreased as it goes away from the principal meridional curve. In the nearsight portion, the radius of curvature is increased and then decreased as it goes away from the principal meridional curve.
In this lens, visual characteristics can be improved to some extent as compared to those disclosed in U.S. Pat. No. 3,910,691. However, in a peripheral region of the farsight portion, in particular, in side regions between the central and lower portions of the farsight portion, a residual astigmatic difference is still considerable. In addition, side regions of the intermediate and farsight portions still suffer from large distortion and fluctuation of an image. Thus, it is yet difficult to obtain a wide field of view.
In the lens disclosed in Japanese Patent Publication No. 63-42764, in order to improve the dynamic vision as well as the static vision, the refracting surface of the lens is divided into three portions of farsight, intermediate and nearsight portions and the manner of distortion of vertical lines is made to conform to the rule of the change of curvature between the centers of the farsight and nearsight portions on the principal meridional curve to lessen distortion of an image in side regions of the intermediate portion. Further, in order to make the distribution of astigmatic differences gentle, horizontal crossing curves consisting of the intersection lines of planes perpendicular to the principal meridional curve and a refracting surface of the lens are defined. In the farsight portion, the curvatures of the horizontal crossing curves are increased in the side regions thereof. In the nearsight portion, the curvatures of the horizontal crossing curves are decreased in the side regions thereof. In the upper portion of the intermediate portion (close to the farsight region), the curvatures of the horizontal crossing curves are once increased, then decreased and again increased while in the lower portion of the intermediate portion (close to the nearsight region), the curvatures of the horizontal crossing curves are decreased and then increased.
In this lens, the dynamic and static visions are improved to some extent as compared to that disclosed in Japanese Patent Publication No. 59-42285. However, from the side regions of the intermediate portion to the side regions of the nearsight portion, in particular, from the side regions of the lower portion of the intermediate portion to the side regions of the nearsight portion, distortion of image is still left. Moreover, the distortion and fluctuation are caused due to distortion increasing abruptly as going away from the principal meridional curve.
As mentioned above, the conventional lenses can secure a visual performance to some extent but it is not sufficient for practical use.